This invention relates to new materials that are useful for the stabilization of organic polymeric material normally tending to deteriorate. In particular, the invention relates to new materials that are useful for the protection of synthetic polymers against the harmful degradative effects, such as discoloration and embrittlement, caused by exposure to light, especially ultraviolet light.
It is known that actinic radiation, particularly in the near ultraviolet region, has a deleterious effect on both the appearance and properties of organic polymers. For example, normally colorless or light colored polyesters turn yellow on exposure to sunlight, as do such cellulosic polymers such as cellulose acetate. Polystyrene discolors and cracks, with an accompanying loss of its desirable physical properties, when exposed to actinic light, while vinyl resins, such as polyvinyl chloride and polyvinyl acetate, spot and degrade. The rate of air oxidation of polyolefins such as polyethylene and polypropylene is materially accelerated by ultraviolet light.
It has been proposed to stabilize polymeric materials against ultraviolet light deterioration by the use of several different types of ultraviolet absorbers. Thus, U.S. Pat. No. 3,004,896 discloses for this purpose 2(2-hydroxyphenyl)benzotriazole derivatives, while U.S. Pat. No. 3,189,630 discloses certain metal salts of hydroxybenzoic acids which are useful as actinic stabilizers in synthetic polymers.
It is known from U.S. Pat. No. 3,705,166 to use monomeric acrylic acid derivatives which contain at least one piperidinyl group having a sterically hindered nitrogen atom as light stabilizers in organic polymers. However, these acrylic acid derivatives are too readily volatile. Furthermore, the possibility of incorporation the monomeric additive in certain substrates is pointed out. However, this has the disadvantage that the polymer structure is destroyed by the additive incorporated and this can lead to a change, which is difficult to regulate, in the characteristics of the substrate to be protected.
A drawback that is common to the use of all such stabilizer molecules is that the small size of such molecules provides them with mobility. This can result in their diffusion to the surface of the polymeric article. On the surface, these stabilizer molecules can be inadvertently removed by mechanical means, resulting in the loss of their stabilizing effect. Also, the comparatively low boiling points of these materials, compared to the temperatures encountered by the synthetic polymers during processing, such as during extrusion, or molding, can as well lead to the loss of the stabilizer molecules. For these reasons, light stabilizer molecules are needed that can be incorporated into polymeric structures, that will produce non-volatile, non-leaching light stabilizers for polymer protection.
To this end, high molecular weight, polyfunctional polymeric light stabilizers have been prepared by incorporating light stabilizing moieties into polymerizable monomeric structures, and then homopolymerizing the resulting compounds to obtain the desired polymeric light stabilizers. Also, these light stabilizer-containing monomeric compounds have been copolymerized with suitable comonomers to produce such polymers. Such teachings are disclosed, for example, in Ger. Offen. No. 2,748,362 and in U.S. Pat. Nos. 4,078,091, 4,205,151 and 4,210,612. However, the particular mechanism of light stabilizing activity of the light stabilizing component in the monomer necessitates careful and limited choice of the system used to prepare the polymers from such monomeric structures.
For example, Ger. Offen. No. 2,748,362 in its Example 1 polymerizes the monomer 4-methacryloyloxy-2,2,6,6-tetramethylpiperidine in the presence of azodiisobutyronitrile (AIBN) at 95.degree. C. to form a polymeric product having a molecular weight of about 5100. Other examples produced copolymers of two different monomeric species, one of which is a hindered acrylic-substituted piperidine, at molecular weights in the range from 1,750 to 9,720, where reported, except in one case, Ex. 38, where a molecular weight of 17,780 was reported. Even so, all of these stabilizing materials possessed relatively low molecular weights, and probably are more aptly characterized as oligomers rather than as polymers. At a molecular weight of 10,000 for example, with a monomer M. Wt. of 500, which is representative, there would only be 20 repeating units in the chain. Many theoreticians believe that this is probably about the minimum number required for the demonstration of polymer characteristics such as, for example, the polymeric Tg.
U.S. Pat. No. 4,205,151 describes polymeric N-substituted maleimides. The polymer may contain as many as 2000 repeating units, and molecular weights may be in the range from about 1,000 to 100,000. These materials are said to function as antioxidants, and to have the advantage of being non-absorbable through the walls of the gastrointestinal tract, to permit use in plastics that may be in contact with foodstuffs.
U.S. Pat. No. 4,210,612 describes homopolymeric and copolymeric light stabilizers characterized by the recurring structural unit, ##STR2## there being N-heterocyclic rings on the side substituents, which may be hindered amines. These polymeric products are said to have good solubility in or compatibility with host polymers, and to manifest a high resistance to extraction. The molecular weights of the stabilizers may be from more than 500 up to 150,000, preferably 500 to 50,000, and most preferably, 1,000 to 20,000, of which the active stabilizing component may be present to a controlled degree.
U.S. Pat. No. 3,705,166 suggests a different approach. Acrylic esters of hindered piperidines, it is said, may be graft-copolymerized to other polymers, or may be copolymerized with other monomers to form polymeric substances having excellent light stability. Basically, however, the patent seems to be concerned with the incorporation of the monomeric acrylic esters of hindered piperidines into synthetic polymers requiring stabilization.
Still another approach to stabilization is described in the Uhrhan et al. U.S. Pat. No. 4,145,512. Permanently stabilized polyurethanes are prepared by reacting with a diisocyanate or with an isocyanate prepolymer a 2,2,6,6-tetraalkyl piperidine derivative having two H-active reactive groups that react with isocyanate groups. The quantity of reactive stabilizer incorporated in the polyurethane may be from 0.05% to 5% by weight based on the polyurethane solids content.
Permanently stabilized polymers are also produced by the process of the Oertel et al. patent, U.S. Pat. No. 4,153,596. Compounds based on 2,2,6,6-tetraalkyl piperidines, that contain one or more reactive groups such as methylol groups, are reacted with polymers that are to be stabilized. The stabilizing radical becomes attached to O- or N-atoms of the polymer. The polymers to be stabilized must themselves be reactive, and contain reactive hydrogens on O- or N-atoms. For example, polymers containing hydroxyl, primary and/or secondary amine groups, amide groups, imide groups, urethane groups or urea groups (in which case the --NH--CO--NH-- group may even be part of a complicated structure), may be used. Those polymers identified as suitably reactive include, for example, polyvinyl alcohol or copolymers of vinyl alcohol, polyamides or copolyamides based on lactams, diamines, dicarboxylic acids or aminocarboxylic acids, copolyamides containing proportions of secondary amino groups (incorporation of diethylene triamine or the like), aromatic or heterocyclic copolyamides, gelatin, (co)polymers of (meth)acrylamide, polyhydrazides, polysemicarbazides or polymers containing --CO--NH--NH--, --O--CO--NH--NH-- or --NH--CO--NH--NH-- groups (optionally as part of complicated structures such as, for example, --N--H--CO--NH--NH--CO--NH), as for example in segmented polyurethane (ureas). The reactive sites of polymers such as these may be present both as part of the backbone chain, as a side chain, and also as terminal groups.
The stabilized polymers of the Oertel et al. patent are generally obtained by adding to the polymers, preferably in solution, from 0.05% to 5% by weight (preferably from 0.1% to 3.0% by weight and, with particular preference, from 0.1% to 2.0% by weight) of the 2,2,6,6-tetraalkyl piperidine compounds containing one or more reactive groups, processing the polymers into shaped articles and reactively attaching the stabilizers to the polymers before, during or after the shaping stage.
The reaction process of the Oertel et al. patent is preferably employed for incorporating reactive 2,2,6,6-tetraalkyl piperidine compounds in and for stabilizing segmented polyurethane elastomers which, in addition to urethane groups, also contain NH--CO--NH groups formed by reaction of isocyanate groups with water and/or compounds containing terminal NH.sub.2 -groups (for example diamines, dihydrazides, carbodihydrazide, semicarbazide hydrazides or hydrazine) and which have a substantially linear, segmented molecular structure, are soluble before shaping in highly polar solvents, such as dimethyl formamide or dimethyl acetamide, and of which the characteristic segments may be characterized by the following formula moiety: EQU --Y.sub.1 --NH--CO--NH--X.sub.1 --NH--CO--NH--,
this segment optionally having been formed from the reaction of an NCO-preadduct OCN--Y.sub.1 --NCO with a chain extender H.sub.2 N--X.sub.1 --NH.sub.2.
As in earlier developments, Oertel et al. had as their purpose the production of permanently stabilized polymers that were used for their own sakes, and not as additives to host polymers to stabilize the host polymers.
A more recent patent, Wiezer et al., U.S. Pat. No. 4,308,362, discloses copolymers of one or more polyalkyl piperidines of the formula ##STR3## with one or more comonomers of the formula ##STR4## the resulting copolymer having a molecular weight of from about 1000 to about 10,000, wherein in the monomers of the formula (I) R.sup.1 is hydrogen or C.sub.1 to C.sub.18 -alkyl, preferably hydrogen or C.sub.1 to C.sub.4 -alkyl and especially hydrogen, R.sup.2 and R.sup.3 are either the same and represent hydrogen or C.sub.1 to C.sub.5 -alkyl, preferably hydrogen or methyl and especially hydrogen, in which case R.sup.4 is methyl. Other formulae are also disclosed. These copolymers are said to be suitable for the stabilization of organic polymers against degradation by light and heat, and are also said to be characterized by low volatility, good migration stability, and good compatibility with the polymers to be stabilized, in spite of their polymeric character.
As in Examples 6 and 7 of this patent respectively, these materials can be formed by ester interchange or by direct reaction with a carboxylic acid-functional polymer, as well as by copolymerization of monomers.
Fu et al., U.S. Pat. No. 4,413,096 is directed to a terpolymer having ethylene monomer units, piperidine substituted acrylate or methacrylate units, and optionally substituted acrylate to methacrylate monomer units.
Offenlegungsschrift No. 27 48 362, relates to copolymers of piperidine and methyl acrylate or methyl methacrylate, rather than piperidine modified acrylate-methacrylate copolymers.